This invention relates to refrigerant compressors and, more particularly, relates to apparatus for protecting the electric drive motor of a refrigerant compressor.
A variety of devices have been produced for the purpose of protecting three-phase AC motors. Some of these devices include the provision of first and second switches arranged so that the first switch will open to disconnect the motor from its voltage source in the event that the second switch fails in the closed position.
Such devices are illustrated in U.S. Pat. No. 2,470,257 (Moore - May 17, 1949) and U.S. Pat. No. 3,656,023 (Hadfield - Apr. 11, 1972). Each of these patents illustrates a protection circuit in which a main or working contactor normally controls the flow of electrical current to a three-phase motor. A safety switch is normally closed and is opened only if the working contactor fails in the closed position. The applicant has discovered that this arrangement has a number of disadvantages which are overcome by the present invention. A normally closed safety switch has a tendancy to stick in the closed position due to ordinary wear and contamination. Contamination may result from insect juices, which are sticky and gluelike. As a result, after a period of time in field service, the safety switch becomes ineffective and useless as a safety switch.
In order to overcome the deficiencies of the prior art, the applicant has discovered that a compressor motor can be safely protected by a single switch and fusible link means connected in series between the motor and a source of AC voltage. If the switch fails in the closed position, the fusible link means are heated in order to switch them to an open circuit condition that disconnects the motor from the voltage source.
Accordingly, it is a principal object of the present invention to provide a switch and fusible link means connected in series between a compressor motor and an AC source, and means for heating the link means if the switch fails in the closed position.
Still another object of the invention is to provide a switching arrangement of the foregoing type in which the fusible link means is contained within a resistor which forms the means for heating so that the heat transfer between the link means and heater means is improved.
Yet another object of the invention is to provide a switching arrangement of the foregoing type in which the resistor that heats the fusible link means comprises PTC material. This feature enables the resistor to be driven from a power source over a wide range of voltages without changing resistor values.
By using a fusible link arrangement of the foregoing type, compressor motors can be protected with a degree of reliability and economy heretofore unattainable.